In: Electrical Engineering
Types of Digital Modulation Techniques:
The most basic digital modulation techniques are constructed based on keying:
PSK is a type of digital modulation technique that conveys data by varying the phase of a fixed frequency reference input (a carrier wave). The modulation is performed by altering the sine and cosine inputs at a specific time. PSK employs a finite amount of phases, each assigned a unique binary digits pattern. Generally, each phase encodes an equivalent number of bits. Each bit pattern forms the word that is described by the specific phase information. The demodulator that is designed especially for the word-set handled by the modulator detects the phase of the signal received and maps it back to the word it describes; thus, the original data is recovered.
It is widely used in the case of wireless LANs, Bluetooth communication, and RFID. At the basic speed of 1 Mb/s, it uses differential BPSK. To extend the speed to 2 Mb/s, DQPSK is used. To obtain 5.5 Mb/s and the full speed of 11 Mb/s, QPSK is used but must be coupled with the complementary code keying.
In general, FSK can be implemented by utilizing fully independent free-running oscillators and switching among them at the start of each period of the word. Usually, autonomous oscillators will not be at the same phase and so the same amplitude at the switch-over instant, creating sudden discontinuities in the transmitted signal.
Practically, many FSK transmitters use only a single oscillator, and the method of switching to a different frequency at the starting of each period of word maintains the phase. The exclusion of discontinuities in the phase (and hence elimination of sudden variations in amplitude) decreases sideband power, decreasing the interference with adjacent channels.
Most of the old telephone lines, modems use frequency-shift keying to send and receive data at rates up to about 1.2 Kilobits per second. The Bell 202 and Bell 103 modems used the FSK technique. Even today, in North America, caller ID uses FSK in the form of the Bell 202 standard. Some old microcomputers used a special form of AFSK modulation.
ASK is a special type of amplitude modulation technique digitally, which represents digital data as quantization in the amplitude of a carrier wave. In an ASK, the binary word "1" is represented by forwarding a fixed-amplitude carrier wave with a fixed frequency for a duration of T (fundamental bit period) seconds. If the signal value is "1," the carrier wave will be transmitted. Else, a signal with value "0" will be transmitted.
Any digital modulation technique uses a restricted number of distinct signals to represent digital data. ASK uses a limited number of amplitudes. Each allocated quanta has a unique pattern of binary numbers. Generally, each amplitude encodes an equivalent number of bits. Each pattern of bits constructs the word that is represented by the specific amplitude. The demodulator terminal, which is designed explicitly for the word-set used by the modulator, recovers the amplitude of the received signal and maps the amplitude back to the word it represents. The phase and the frequency of the carrier wave are kept constant.
ASK is a traditional modulation scheme used in digital communication for a huge number of low-frequency Radio Frequency applications.
QAM belongs to the family of digital modulation techniques but mostly related to analog modulation techniques extensively used in advanced telecommunication systems to transmit information. It conducts two analog message signals, or two bit-streams or digital-word, by modulating the amplitudes of two carrier signals, using amplitude shift keying scheme or amplitude modulation (analog) technique. The two carrier waves of the same frequency with a phase difference of 90°. A condition, commonly known as quadrature or orthogonality. The transmitted signal is produced by adding the two carrier signals together. At the receiver, the two signals can easily be separated or demodulated because of their property of orthogonality.
QAM is used widely as a modulation technique for digital telecommunication systems, like in 802.11 Wi-Fi types. Arbitrarily high spectral efficiencies can be obtained with QAM by introducing a proper constellation size. QAM is widely used in optical fiber communication systems. As the rate of bit increase, QAM64 and QAM16 can be used.